This invention relates to metal oxide varistors and, more particularly, to a method of achieving a more homogeneous mixture of the several components prior to pellet pressing and thus to provide improved devices.
In general, the current flowing between two spaced points is generally directly proportional to the potential difference between those points. For most known substances, current conduction therethrough is equal to the applied potential difference divided by a constant, which has been defined by Ohm's law to be its resistance. There are, however, a few substances which exhibit non-linear resistance. Some devices, such as metal oxide varistors, utilize these substances and require resort to the following equation (1) to quantitatively relate current and voltage: EQU I = (V/C).sup..alpha. (1)
where V is the voltage applied to the device, I is the current flowing through the device, C is a constant and .alpha. is an exponent greater than 1. Inasmuch as the value of .alpha. determines the degree of non-linearity exhibited by the device, it is generally desired that .alpha. be relatively high. .alpha. is calculated according to the following equation (2): ##EQU1## where V.sub.1 and V.sub.2 are the device voltages at given currents I.sub.1 and I.sub.2, respectively.
At very low voltages and very high voltages metal oxide varistors deviate from the characteristics expressed by equation (1) and approach linear resistance characteristics. However, for a very broad useful voltage range the response of metal oxide varistors is as expressed by equation (1).
The values of C and .alpha. can be varied over wide ranges by changing the varistor formulation and the manufacturing process. Another useful varistor characteristic is the varistor voltage which can be defined as the voltage across the device when a given current is flowing through it. It is common to measure varistor voltage at a current of one milliampere and subsequent reference to varistor voltage shall be for voltage so measured. The foregoing is, of course, well known in the prior art.
Metal oxide varistors are usually manufactured as follows. A plurality of additives is mixed with a powdered metal oxide, commonly zinc oxide. Typically, four to twelve additives are employed, yet together they comprise only a small portion of the end product, for example less than five to ten mole percent. In some instances the additives comprise less than one mole percent. The types and amounts of additives employed vary with the properties sought in the varistor. Copious literature describes metal oxide varistors utilizing various additive combinations. For example, see U.S. Pat. No. 3,663,458. A portion of the metal oxide and additive mixture is then pressed into a body of a desired shape and size. The body is then sintered for an appropriate time at a suitable temperature as is well known in the prior art. Sintering causes the necessary reactions among the additives and the metal oxide and fuses the mixture into a coherent pellet. Leads are then attached and the device is encapsulated by conventional methods.
A problem encountered in the manufacture of metal oxide varistors by the prior art method is the inability to precisely predict and control the properties of the device. Thus manufacturing yield is a matter of concern to varistor manufacturers. This inability to control the properties of the device becomes more severe as the manufacturing process is changed to one which should theoretically yield lower voltage devices and has heretofore frustrated efforts to develop a commercially suitable low voltage (e.g. 80 volts) device.
While the conduction process in metal oxide varistors is not fully understood, it is believed that an important part of the problem is the inability to thoroughly and uniformly mix the several components prior to pellet processing. The reasons for this belief are as follows. It must first be realized that chemical changes occur during sintering. When several additives are used to tailor the resulting properties of the device, it appears desirable that they interact in addition to each reacting with the zinc oxide. However, often each additive comprises only a fraction of a mole percent of the total compound mixture and the metal oxide comprises 90 or more mole percent. Thus, inasmuch as the components are in a particulate form, the final mixture just prior to pellet pressing is a dispersion of isolated particles of each additive in a sea of metal oxide particles. Consequently, many of the additive particles are surrounded by an abundance of metal oxide particles but may be remote from particles of the other additives and thus be unable to react therewith. An additive particle reacting only with the metal oxide may, of course, create a different material than an additive particle reacting with other additives and the metal oxide. This situation is more likely to occur if the mixing of the metal oxide and the additives is inadequate. Thus, the inability to adequately control the mixing of the components makes final varistor performance difficult to control and has frustrated the development of low voltage devices.
One method of partially alleviating the problem is to increase the additive concentration. However, this is often an unacceptable solution because of the high cost of certain additives and the effects on the varistor properties that an increase in additive concentration can have.
It is an object of this invention, therefore, to provide a homogeneous mixture for varistor fabrication and to insure proper interreaction of the additives.
Another object of this invention is to provide a metal oxide varistor of the foregoing character having good low voltage characteristics.